The aim of this proposal is to design and develop new and practical avalanche photodiode (APD) based detector modules, with depth of interaction (DOI) encoding, suitable for use in a small PET scanner dedicated to breast imaging. Many important detector and system configuration questions will be studied in this project. The work in this proposal will provide the foundation for our ultimate goal of developing a compact clinical PET system for breast imaging that provides images of significantly improved quality at substantially lower cost compared with current commercial PET systems. This will extend the application of PET in the diagnosis and management of patients with breast cancer. The proposed detector module will consist of an array of small discrete lutetium oxyorthosilicate (LSO) scintillator elements, optically connected to two APD arrays at both ends. The newly developed APD array has high quantum efficiency, uniform and high gain among all channels, and is compact in size with small dead space. This new detector module will provide high spatial resolution, high detection efficiency, excellent count rate performance, and sufficient depth of interaction resolution to improve the uniformity of spatial resolution across the field of view. We have previously demonstrated the feasibility of the detector design. In this proposal, we will first optimize the detector performance by carefully studying the effects of crystal surface treatment, external reflector and couplings of different components, and pay close attention to the trade-off among energy, timing, spatial and DOI resolution. We will optimize the operating conditions of the APD array, and use parallel readout electronics to read out each detector channel in order to reduce noise levels. We will construct and evaluate two full detector modules to assess the detector design concept, as well as the importance of the DOI measurement capability. Finally, the two detectors will be mounted on rotation/translation stages to perform imaging experiments with different geometry (ring and rotating plate), allowing the performance of an entire PET system to be projected.